Exposure to ultra violet radiation can cause a wide range of deleterious effects in human skin, including dehydration, burning, premature aging, and discoloration. In addition, it has been widely accepted and documented that excessive exposure to ultraviolet radiation can be carcinogenic in humans. Exposure to ultraviolet radiation is of a particular concern for individuals that spend a significant amount of time outside and exposed to the sun, and those having fair complexions and who are less tolerant to the sun's ultraviolet rays. In addition, damage to the earth's ozone layer and its ability to filter out ultraviolet radiation has further raised the concern over the harmful effects that exposure to ultraviolet radiation may have.
While sunscreen preparations are, to varying degrees, reasonably effective in filtering out harmful radiation and limiting the skin's exposure to the sun, they are limited with respect to their duration of usefulness. Commercially available sunscreen preparations are rated on what is commonly referred to as a sunscreen protection factor (SPF) scale. Generally the higher the SPF factor the greater the sunscreen protection that is provided. However, often individuals are unaware of the intensity of the sun's rays during a particular time of day, or with changing weather conditions, and do not realize that sunscreen having a certain SPF factor may not protect them adequately throughout the day. Moreover, differences in climate, sunscreen formulations, activity and natural characteristics of the skin of different individuals means that the effectiveness of sunscreens may vary dramatically for the different individuals. There exist 5 skin types that differ according to the color of human hair, eyes, and skin, and by their reaction to overexposure to UV radiation. The permissible time for exposure to UV radiation on a mid-summer day changes from 15 minutes for skin type no. 1 to about 2 hours for skin type no. 5 (without using sun screen).
The dangers presented by overexposure to UV radiation makes it desirable for a user to have a device to monitor his or her exposure to UV radiation. UV indicators and dosimeters using photochromic materials are known. Photochromic materials, for example, photochromic dyes and pigments, change color upon exposure to activating light, in this case UV light. In many cases, the unactivated, original material is colorless or clear. As the activated forms of many photochromic dyes are somewhat unstable, the amount of degradation of the dye, determined for example, by a decrease in the color intensity of the activated form, can be used as a measure of the amount of UV exposure. Thus, a simple, semi-quantitative dosimeter may be prepared using photochromic materials.
For example, U.S. Pat. No. 4,829,187 to Tomita et al describes a dosimeter which uses (a) a photo activator to form free radicals upon irradiation with UV rays, (b) a coloring agent which exhibits a color change in the visible region of a spectrum through action of the free radicals; (c) a UV absorber that controls the amount of free radicals generated from the photo activator so as to enable measurement of the irradiated dose; and (d) a polymer matrix within which the constituents are contained.
Other patents and patent publications describe dosimeters which, in addition to a photochromic material, comprise a reference material that allows a user to compare the color change of the photochromic material with a color that has been predetermined to be the color that the photochromic material would be when exposed to a predetermined amount of UV radiation. Thus, U.S. Pat. No. 4,705,046 discloses a device for the qualitative measurement of UV radiation received by the skin which consists of a laminar strip comprising a photochromic material and a reference material. The reference material is suitably a dye which matches closely the color of the photochromic material after exposure to a pre-determined quantity of UV radiation. One can visually assess the exposure to UV radiation by a direct comparison of the color densities of the photochromic material and the reference material during exposure to sunlight.
Since photochromic materials often revert to their original color when the activating light is removed, other patents describe the use in a dosimeter of components, in addition to a photochromic material, to inhibit the photochromic material from reverting to its original color, i.e., to make a color change of the photochromic material irreversible. For example, U.S. Pat. No. 8,961,897 to Faran describes the distribution within a matrix, along with a photochromic compound, of a color changing agent that reacts with the photochromic compound to form a complex capable of irreversible color change after it has been exposed to a predetermined UV radiation. The particular combination of a photochromic compound, the color changing agent compound and the type of the matrix used in the measuring device is chosen so that the measuring device changes color during exposure to a predetermined dose of UV radiation according to the application, for example the dose which exceeds an individual user's permissible MED corresponding to his or her personal skin type. The particular efficacy is defined for a particular skin type, by virtue of this provision the user can choose the measuring device which is safest for user and thus avoid damage to the user's skin.
Other similar devices to those described above are found, by way of example, in U.S. Pat. Nos. 6,132,681; 6,818,904; 5,986,273; 5,589,398; 3,787,687, 5,117,116, 4,659,649, 5,296,275, 3,903,423, 5,581,090; 6,734,440, 5,612,541, and 6,504,161; and in US Pat App Pub 2001/0019110, 2004/0109789, 2002/0022008, 2012/0137958, 2005/0285050 and 2012/0288690. The disclosures of these and the other patents and patent application publications described above and throughout this specification are incorporated herein in their entirety by reference.
To make a dosimeter convenient for personal use, the prior art describes the possibility of incorporating a dosimeter into a shape that may be worn on the body or clothes of a user. Thus, U.S. Pat. No. 5,914,197 to Goudjil describes an ultraviolet active wristband comprising a clear ink composition comprised of an ink vehicle used in screen printing and a reversible photochromic compound. The ink composition is applied to a substrate and formed into the shape of an adjustable wristband.
The '897 patent to Faran describes a dosimeter that may be worn by a user as a sticker or wristband and may be calibrated to work simultaneously with a sun screen that is applied to the device surface, and thus to increase the permissible time of exposure to the UV radiation. Reference is made in this respect to FIG. 1 of the drawings, which shows a prior art device comprising a polymeric matrix layer 10 atop an opaque layer 12 that is present to provide a homogenous background, and an optional third layer 14 that is made of a sticky material that can be attached to the user's skin, clothing or equipment.
The matrix 10 comprises an active photochromic compound 16 and a color changing compound 18 distributed therein. The matrix is described as being provided in order to carry the active photochromic compound 16 and to reliably protect it from corrosion due to ambient humidity and to mechanical impact. However, in the prior art device, there is nothing to protect the matrix itself from corrosion caused by sunscreen that may be applied to a surface of the matrix.
In particular, the '897 patent does not consider that the components typically found in sunscreens, including titanium dioxide and zinc oxide, may be photoactive, particularly when present as nanoparticles. They have the ability to absorb UV radiation, and in the presence of moisture convert water molecules into chemically active hydroxyl free radicals. These highly reactive chemicals are highly corrosive and, if the sunscreen comes in contact with a surface of a polymer matrix comprising a photochromic material, could etch the surface resulting in the sunscreen becoming a permanent part of the matrix material. This could prevent UV radiation from coming in contact with the photochromic material and thereby prevent a color change from occurring.
Accordingly, what is needed is a dosimeter device for use with sunscreen that has a layer of material that is preferably capable of absorbing a minimal amount of sunscreen but that nevertheless protects a matrix/photochromic layer of the dosimeter from contact with the sunscreen so absorbed. Preferably, the device is flexible enough such that, notwithstanding its multi-layer construction, it can be shaped for use as a wristband that may be comfortably worn on the wrist of a user and that has a smooth bottom layer that does not irritate the skin of the user. Preferably, also, the device can be made in a simple and cost-effective manner.